Overall objectives of the total project are to characterize specific conditions in which the temperature and circulatory regulatory systems interact and compete with each other and to describe the physiological mechanisms by which adjustments are made to satisfy potentially incompatible demands from the two systems. A specific objective has been to determine how modifications in blood volume and/or osmolarity affect cardiac filling pressure during exercise in the heat and, further, how differential control of the cutaneous circulation contributes to these effects. Our immediate goal has been to describe the partition of the cardiac output during exercise in the heat, particularly in cases where cardiac filling pressure is compromised. We have used dehydration, venesection and different postures to manipulate venous return. Since increased skin blood flow is essential to augment heat transfer from core to skin during exercise and since increased skin blood flow and peripheral venous pooling compromise venous return and muscular perfusion in conditions of high demand, it is necessary to determine how the body is able to meet the various demands from muscle and skin. With a knowledge of the mechanisms the body employs to maintain cardiac output in different conditions, it becomes possible to better understand adaptive mechanisms which accompany physical training and/or heat acclimation.